Early luteal phase progestin concentration influences endometrial function in pregnant mares

In the horse, it is still unclear if and to what extent low progestin concentration contributes to early conceptus loss. In the present study, we have investigated if reduced or elevated progestin concentration in the early luteal phase influences endometrial function and conceptus development. We hypothesized that reduced progestin concentration via delayed downregulation of endometrial progesterone receptors (PR) influences endometrial function in healthy fertile mares while progestin substitution does not. Genitally healthy estrous mares (n = 8; age 4-14 years) were inseminated and treated with either altrenogest (0.044 mg/kg once daily orally) on days 5-10 after ovulation (ALT), cloprostenol (125 μg once daily intramuscularly) on days 0-3 after ovulation (CLO) or left untreated (CON). ALT and CLO treatment were chosen to increase and decrease total peripheral progestin concentration, respectively. Each treatment was given to every mare in consecutive cycles. On day 14 after ovulation, endometrial fluid was collected with a cotton roll inserted into the uterus and an endometrial biopsy for immunohistological demonstration of progesterone (PR) receptor distribution was collected. In endometrial fluid, free amino acid concentrations were analyzed by ion exchange liquid chromatography with an amino acid analyzer. Cell nuclei staining positive for the PR were determined in the luminal and glandular epithelium as well as in the stroma. Pregnancy rate tended to differ among treatments. The percentage of luminal epithelial cells staining positive for PR differed among treatments (p < 0.05) and was higher in CLO (84.1 ± 1.9%) than in ALT (70.7 ± 4.7%) and CON cycles (72.8 ± 4.1%). Concentrations of the amino acids isoleucine (CON 0.17 ± 0.03, CLO 0.14 ± 0.02, ALT 0.23 ± 0.04 μmol) and lysine (CON 0.27 ± 0.08, CLO 0.18 ± 0.05, ALT 0.44 ± 0.13 μmol) were influenced by treatment (p < 0.05) and lower in CLO than in ALT and CON cycles. In conclusion, impaired luteal function due to CLO treatment during the early luteal phase of pregnant mares delayed downregulation of progesterone receptors in the endometrial epithelium on day 14. This influenced endometrial function as reflected in lower concentrations of the amino acids lysine and isoleucine in endometrial secretions. Enhanced progestin concentration had less clear effects in healthy fertile mares.